This invention relates to a final texture annealing cycle to promote improved secondary recrystallization. Particularly, the invention relates to a substantially isothermal anneal at a selected recrystallization temperature.
In the manufacture of grain-oriented silicon steel, it is known that if improved secondary recrystallization texture, e.g., Goss texture (110)[001], is achieved, the magnetic properties, particularly permeability and core loss, will be correspondingly improved. The Goss texture (110)[001], in accordance with Miller's indices, refers to the body-centered cubes making up the grains or crystals being oriented in the cube-on-edge position. The texture or grain orientations of this type refers to the cube edges being parallel to the rolling direction and in the plane of rolling, and the cube face diagonals being perpendicular to the rolling direction and in the rolling plane. As is well known, steel having this orientation is characterized by a relatively high permeability in the rolling direction and a relatively low permeability in a direction at right angles thereto.
The development of a cube-on-edge orientation is dependent upon a mechanism known as secondary recrystallization. During recrystallization, secondary cube-on-edge oriented grains are preferentially grown at the expense of primary grains having a different and undesirable orientation. The steel composition, particularly the impurity contents, the processing operations including hot rolling and the degree of deformation in each cold-rolling operation, intermediate and final continuous annealing time and temperature cycles, and the final texture annealing procedure must all be carefully controlled to attain the optimum texture development. A steel that has not obtained optimum texture development may have a substantially uniform but inadequate grain size and structure and resulting poor magnetic properties or may exhibit a "banding" of inferior grain structure. Generally, banding means areas or bands of inferior grain structure extending across the width of the coil surrounded by areas of well-textured steel. Generally, the initial phases of secondary recrystallization occur at about 1550.degree. F. (843.degree. C.), however, secondary grain growth proceeds much faster and more efficiently at temperatures of about 1600.degree. F. (871.degree. C.) or more. The operation through which the secondary grains are preferentially grown and consume the primary grains is known as final texture annealing.
In the manufacture of grain-oriented silicon steel, the typical steps include subjecting the melt of 2.5-4% silicon steel through a casting operation, such as a continuous casting process, hot rolling the steel, cold rolling the steel to final gauge with an intermediate annealing when two or more cold rollings are used, decarburizing the steel, applying a refractory oxide base coating to the steel, and final texture annealing the steel, such as in a hydrogen atmosphere, to produce the desired secondary recrystallization, and purification treatment to remove impurities, such as nitrogen and sulfur. The final texture annealing is typically performed at a temperature in excess of 2000.degree. F. (1093.degree. C.) and held for an extended time period of at least 4 hours and generally longer to remove impurities.
A typical thermal cycle of the final texture annealing practice may include a reasonably continuous heating rate of approximately 50.degree. F./hour (27.8.degree. C./hour) from the charge temperature of the coated strip to a temperature high enough to effect purification. The charge temperature in mill practice, typically, is on the order of room temperature of 80.degree. F. (26.7.degree. C.) or more and the purification temperature may range from 2000.degree. F. (1093.degree. C.) up to a maximum of about 2300.degree. F. (1260.degree. C.) and preferably up to 2250.degree. F. (1232.degree. C.). The steel is generally subjected to a soaking at the purification temperature to remove the impurities for a long time, typically on the order of about 20 hours at or higher than 2100.degree. F. (1150.degree. C.).
Numerous attempts by others have been made to improve the final texture. U.S. Pat. No. 2,534,141--Morrill et al discloses a two-stage final texture anneal to improve the orientation. First, the decarburized sheet is held for 4-24 hours at 850.degree.-900.degree. C. (1560.degree.-1650.degree. F.), and preferably at 875.degree. C. (1605.degree. F.), in a reducing or nonoxidizing atmosphere to encourage and permit nucleation of well-oriented crystals and their growth. Second, the steel is then held at a temperature at 900.degree. to 1200.degree. C. (1650.degree.-2192.degree. F.), and preferably 1175.degree. C. (2147.degree. F.) in a reducing atmosphere to permit completion of the growth of the well-oriented crystals and to relieve mechanical strain.
U.S. Pat. No. 4,157,925--Malagari et al discloses a process for producing a cube-on-edge orientation in a boron-inhibited silicon steel. The process includes heating the steel from a temperature of 1700.degree. to 1900.degree. F. (926.degree. to 1038.degree. C.) at an average rate of less than 30.degree. F./hour (16.7.degree. C./hour) so as to provide a minimum time period for the selective grain-growth process to occur and to final texture anneal the steel by heating to a temperature in excess of 2000.degree. F. (1093.degree. C.) and to a maximum temperature of 2300.degree. F. (1260.degree. C.) for purification of the steel.
U.S. Pat. No. 4,318,738--Kuroki et al discloses in Example 3 a method for producing grain-oriented silicon steel containing aluminum wherein the decarburized and coated sheet is heated up to 900.degree. C. (1650.degree. F.) in a 75% H.sub.2 and 25% N.sub.2 atmosphere with a heating rate of 20.degree. C./hour (36.degree. F./hour), then heating between 900.degree. to 1050.degree. C. (1650.degree.-1922.degree. F.) in the same atmosphere at a heating rate of 5.degree. C./hour (9.degree. F./hour), between 1050.degree. and 1200.degree. C. (1922.degree.-2192.degree. F.) in 100% H.sub.2 atmosphere at a heating rate of 20.degree. C./hour (36.degree. F./hour) where the steel is maintained at 1200.degree. C. (2192.degree. F.) for 20 hours in the 100% H.sub.2 atmosphere.
None of these patents disclose the present invention. What is needed is an improved final texture annealing process wherein improved cube-on-edge orientation of the secondary grains may be achieved during secondary recrystallization to result in improved permeability and core loss values. The improved final texture annealing process should include control of the heating cycle and result in improved productivity as measured by an overall improvement in quality.
It is known that variations occur in magnetic properties within a given coil of silicon steel. The variations can be measured by taking samples from the coil ends and measuring the core loss values of those samples. A convenient measure of quality improvement is the percentage of coils having a poor end core loss at 60 Hz equal to or less then 0.714 WPP at 17 KG (1.57 WPKg at 1.7 Tesla). It is desirable to improve productivity so that an increasing percentage, and preferably the majority, of the coils produced satisfy minimum core loss values, such as that above.
It is also an objective to develop a process which substantially eliminates the "banding" problem.